Category: CW keying issue

Bob N1KW has identified the cause of the brief glitch on going into TX on CW with the µBITx. While analysing the circuit Bob realised that the large capacitor C52, which is charged during receive, would feed back through R52 and R18 keeping the receive path (Q10, 11, and 12) after the balanced modulator “hot” for a brief period. When the transmit path is activated, the receive side of the circuit is going to remain on for some period of time due to the time constants of C52 and its loads. It is understandable that if both directions of the circuit are on, it could oscillate during that time!

To resolve the issue he simply added a diode in series with R52 (cathode toward C52) so that C52 can no longer back feed power to Q10, 11, and 12 upon initiation of transmit state. Now the transmitter output looks perfectly clean on the spectrum analyzer at beginning of TX. Shorting the diode causes the problem to show as before.

The same issue could apply to C64 when transitioning from TX to RX but at least there will not be spurious emissions going out over the air. He plans to add a diode in series with R66 in the same manner just for fun.

Note that in KD8CEC firmware you can add a keying delay to fix this issue in firmware.

Allan VK2GR was concerned to reduce key clicks and improve the shape of CW from the µBITx.

Being mainly a CW operator, he was concerned about the V3 board uBitx transmit CW wave shape being very hard – almost a square waveshape. As expected, key clicks could be heard on either side of the signal. A look at the circuit diagram and a little tinkering has vastly improved the situation.

The photo above shows where a short wire was soldered to the hot side of C1 for the tests. In reality the wave shape is now a little soft on the trailing edge, so 0.047 or 0.68uf may be sufficient for some people. More work could possibly be done with the CW keying RC network to the 1st balanced mixer, however this one component simple fix will suit my needs.

All that was required was to increase the value of C1 from 0.1uf to 1uf. Below are some oscilloscope photos showing the end result of improvements to the leading and trailing edge of the transmitter output using a 1uf connected across C1.

John AD0RW who has studied the CW keying issue has determined that the primary problem is that the analogue to digital conversion in the µBitx was having trouble distinguishing between “manual key down” and “dit”. This causes keying errors. For example, an ‘I’ becomes an ‘N’ when the dit paddle is held closed.

John incorporated the keyer code from W0EB and W2CTX into his personal software build, but he was determined to save the last analog input for S/power metering. So he kept the single input that detects four different levels. Actually, he doesn’t care much about straight keying so he left out the manual key resistor.

When looking at the nominal voltage levels with the provided resistors, he observed that there was only around 0.22 volts between the “dit” and “manual key” levels (1.60 vs 1.38 V). On the other hand, there is 1.8 volts between “dit” and “dah”. Errors due to fluctuations would be much more likely between “dit” and “manual key” levels.

He investigated options for resistor replacement, and in the end, replaced the 2.2k resistor with a 5.1k one. Now the nominal levels are 3.4 V for “dah”, 2.6 V for “dit”, and 2.1 V for “both”. The boundary ADC values were adjusted in the software.

He has found the results to be favourable so far in his testing, including sending a fair amount of practice code at speeds up to 25 wpm. The iambic action seems flawless and smooth.

He notes that “I might actually get good at sending iambic style someday…”. Some of us need to try this solution. Saving ports is a good idea on an Arduino Nano! It would also be helpful to know what values folk are using for the thresholds for the boundary points in the sketch.